The microdistribution of larvae and pupae of three species of Simuliidae (Odagmia variegata, O. obreptans and Simulium rupicolum) has been observed in different streams in the Pyrenees.

These species occur where there is a high water velocity which, in the streams studied, was present in zones with a torrential discharge. We have not found them in zones with a general discharge of the fluvial type. Smooth stones with a diameter greater than about 10 cm are the most frequently colonised substrata. The surfaces facing upstream and slightly inclined carry the greatest densities of larvae. In fast flowing zones with a longitudinal gradient of a reduction in velocites, the larvae are concentrated in a narrow band where the velocity is highest. The body of the larva lies in a downstream direction and moves with the pulsations of the current.

The larvae of Simuliidae rapidly colonise channels immersed in the stream (fig. 2); and therefore it is possible to define the hydrological conditions at the places where the larvae are attached.

The flow varies with time at the same station and the discharge can change from a fluvial type to a torrential type (fig. 3 and 4).

The profile of currents at the surface, that is to say in the zone where the invertebrates are living, can be determined as a function of the relative roughness of the surface (fig. 6).

The presence of a laminar film (laminar sub-layer) depends upon the strength of the current and the unevenness of the substratum (fig. 7 and 8). For a given current speed, the relationship between the thickness of the laminar film and the index of roughness or unevenness affects the possible formation of a laminar film (fig. 8).

The variation in the thickness of the laminar film is shown as a function of current speed in a graph (fig. 9) which also takes into account the conditions caused by index of roughness.

The laminar film has only a small influence on the living conditions of larvae of Simuliidae whilst turbulence is an important factor.

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